This invention relates to gas phase exothermic reactions to make particulate product in fluidized bed reactors. The invention is described with respect to olefin polymerization but is not limited to the production of polymeric products; rather, it may be practiced in connection with any exothermic reaction which is carried out in a gas fluidized bed with external cooling. It relates particularly to improvements in the condensing mode of operation in which a portion of the fluidizing gas or fluid is withdrawn from the reactor, cooled to remove the heat of reaction, partially condensed, and recycled back to the fluidized bed reactor. In the present invention, the recycle is effected by splitting the recycle stream in at least two streams directed to different areas of the reactor.
The gas phase fluidized bed process for polymerization permits a reduction in energy requirements and capital investment compared with more conventional processes. However, a limiting factor is the rate at which heat can be removed from an exothermic reaction occurring within a fluidized bed. The heat of reaction is commonly removed from the fluidized bed by compression and cooling of a recycle stream external to the reactor vessel. The circulated recycle stream promotes fluidization of the bed within the reactor. The fluid velocity within the reactor is limited by the need to prevent excessive entrainment of solids in the fluidizing gas stream as it exits for recycle from the reactor. Hence the amount of fluid which can be circulated and cooled per unit of time to remove the exothermic heat of polymerization is limited. As polymer product is produced and removed from the fluidized bed, reactants and catalyst material are continuously supplied either to the recycle stream or directly to the reaction zone of the fluidized bed.
The quantity of polymer exothermically produced in a given volume of the fluidized bed is related to the ability to remove heat from the reaction zone. Adequate heat removal is critical to maintain a uniform temperature within the fluidized bed and also to avoid catalyst degradation and polymer agglomeration. The temperature in the reaction zone is controlled below the fusing temperature of the polymer particles. The dew point of the recycle stream is the temperature at which liquid condensate begins to form in the recycle stream. By cooling the recycle stream below the dew point temperature and then injecting the two phase mixture thus formed into the reaction zone, the heat of vaporization of liquid is available to consume a portion of the exothermic heat of polymerization. This process is known as xe2x80x9ccondensed modexe2x80x9d operation of a gas phase polymerization process. As disclosed by J. M. Jenkins et al. in U.S. Pat. Nos. 4,543,399 and 4,588,790 and by M. L. DeChellis, et al. in U.S. Pat. No. 5,352,749, operation in xe2x80x9ccondensed modexe2x80x9d permits an increase in the space time yield of the reaction systemxe2x80x94that is, an increase in the amount of polymer produced per unit of time in a given fluidized bed reactor volume.
Below the reaction zone of the fluidized bed is a gas distributor grid plate. Its function is to provide a uniform distribution of the recycle stream into the bottom of the bed. Below the gas distributor grid plate is located a bottom head mixing chamber where the recycle stream is returned after being compressed and cooled. As disclosed by S. J. Rhee, et al., in U.S. Pat. No. 4,933,149, flow deflection devices can be designed and positioned within the bottom head mixing chamber, to avoid excessive build up of entrained solids within the bottom head mixing chamber when operating without partial condensation of the recycle stream. When operating in xe2x80x9ccondensed modexe2x80x9d, a deflector geometry as disclosed in the ""149 patent may be used to avoid excessive liquid flooding or frothing in the bottom head mixing chamber. However, as the condensing level is increased to further enhance heat removal and space time yield, excessive amounts of liquid can exist in the bottom head mixing chamber. This can lead to liquid pooling and instability problems.
The fluidized bed discharge process described by Aronson in U.S. Pat. No. 4,621,952 is an intermittent semi-batch process involving the transfer, by pressure differential, of solid and gas through multiple vessels. Being semi-batch in nature, the product removal capacity of a given facility is constrained by the time duration of the steps necessary to complete the process. The Aronson discharge process includes interconnecting conduits with valves between the vessels to permit gas venting and pressure equalization. The gas contains valuable raw materials for the fluidized bed reaction system. The gas may include unreacted monomers and comonomers; inert materials are also common. Aronson discloses that the discharge process obtains the desired transfer of solid material while minimizing gas losses. Aronson does not, however, monitor liquid in the product discharge tanks or inject fluid to a point higher than product withdrawal.
As disclosed by Jenkins, et al., in U.S. Pat. No. 4,543,399 and by Aronson in U.S. Pat. No. 4,621,952 the polymer product is intermittently withdrawn from the fluidized bed at an elevation above the gas distributor grid plate. At increasing levels of partial condensation of the recycle stream the likelihood increases that undesirably high levels of liquid phase may exist in lower portions of the fluidized bed. Unfortunately during a product discharge event liquid can be carried out of the reactor along with the granular polymer and gases. Because of the depressurization which takes place during product discharge, the liquid expands and vaporizes, which may cause temperature reduction and pressure elevation within the discharge equipment. This can reduce the fill efficiency of the discharge system, and the reduction in fill efficiency in turn reduces the production capacity by increasing the time to depressurize, and increases the raw material usage of the process. Accordingly, it has been difficult to increase the liquid content in the recycle stream to enhance the efficiency of removing the heat of reaction.
In Chinh et al, in U.S. Pat. No. 5,804,677, the patentees assert they describe the separation of liquid from a recycle stream; the separated, collected liquid is injected into the fluidized bed above the gas distributor plate. The present invention also injects recycle liquid above the distribution plate, but applicants"" liquid is handled as a liquid/gas mixture and as a more or less predetermined fraction of the recycle stream, as a slip stream, divided simply and directly in the recycle conduit. Because of the applicants"" manner of separating, we are able to enhance the ratio of liquid to gas in the slip stream as compared to the withdrawn recycle stream, and thus simply and directly, without additional or special equipment, improve heat exchange efficiency and enhance the space/time yield of the process. In addition, we are able to optimize the product discharge cycle by coordinating liquid volume in the discharge tanks with the rate of injection of liquid above the distributor plate.
Our invention comprises splitting the recycle stream, after compression and cooling, into at least two streams. One of the streams is returned to the distributor grid plate or similar device below or near the bottom of the bed and the other(s) are returned to the fluidized bed at one or more points above the distribution grid plate. The stream is split by a conduit segment designed for the purpose, sometimes herein called a splitter.
Preferably, the recycle stream is divided into two streams, the smaller of which is 5 to 30 percent of the total recycle stream and contains an enrichment of the liquid portion as a function of the relative momentums of the liquid and gas components of the recycle stream, impacting in the splitter, the liquid droplet size, and the particular configuration of the splitter. The liquid content (percentage by weight) of the smaller stream is preferably enriched to a percentage 1.01 to 3.0 times, more preferably 1.1 to 2.5 times that in the stream prior to separation. The larger of the separated streams, having a lower liquid concentration but a higher volume, is recycled to the bottom head mixing chamber of the reactor vessel and introduced into the reaction zone in a uniform fashion more or less conventionally through a gas distributor grid plate. The smaller stream or streams having an enriched liquid phase, is (or are) recycled into the reaction zone at an elevation above the gas distributor plate. Because of the lower ratio of liquid to gas in the larger stream as compared to the original cooled/condensed stream, only a minimal disturbance of the fluidized bed is imparted. We are thus able to inject higher quantities of recycled liquid into the bed without causing difficulties in the product withdrawal system.
An attractive novel feature of our modified recycle technique is that the separation of the recycle stream may be conducted without using mechanical equipment such as separators, hydrocyclones, demisters, scrubbers, entrainment collection devices, pumps, compressors or atomizers. Rather, by withdrawing the small two phase stream or streams from the recycle piping line, by the use of an elbow, bend, tee, or other piping configuration, an enrichment occurs of the liquid content in the small stream. This occurs without any moving parts or the application of energy. This enrichment is due to the difference in momentum between the lower density vapor phase and the higher density liquid phase. As a result of inertia, the liquid droplet trajectories deviate from the streamline of the bulk vapor flow. The liquid phase may exist in the form of droplets ranging in size from 50 to 2000 microns. By selection of a suitable piping system, the small stream or streams, which have been enriched in liquid content, may be re-injected into the reaction zone of the fluidized bed at a location above the gas distributor grid plate, preferably above the product withdrawal level. In this manner a large quantity of the condensed liquid exiting the cooler can be injected into the upper portions of the fluidized bed without separating the gas and liquid phases using mechanical equipment. This is an advantage over the methods disclosed by Chinh, et al. in U.S. Pat. Nos. 5,541,270, 5,668,228, 5,733,510 and 5,804,677 (see the summary above) in that the financial costs for mechanical equipment such as separators, hydrocyclones, demisters, scrubbers, entrainment collection devices, pumps, compressors or atomizers are not incurred. Some of these have moving parts and all entail substantial maintenance problems. The advantage compared to conventional technology incorporated by Union Carbide Corporation and disclosed by Jenkins, et al. in the U.S. Pat. No. 4,543,399 and U.S. 4,588,790 is that the liquid re-injection into the reaction zone of the fluidized bed can occur at one or more points above the gas distributor grid plate without substantial disturbance of the fluidized bed.
We use the term xe2x80x9csplitterxe2x80x9d to mean an elbow, bend, tee, or other conduit segment having an inlet (upstream) portion and two or more outlet (downstream) portions. The outlet portions may be configured, either by a reduction in overall internal diameter or by one or more obstructions or diversions, to provide a resistance to the flow of fluid, which will, to at least some degree, cause liquid to coalesce or accumulate in at least one of the exit portions. Preferably, the incoming fluid is divided into a primary stream containing a high ratio of gas to liquid compared to the secondary stream(s) and at least one secondary or slip stream containing a relatively high ratio of liquid to gas compared to the fluid entering the splitter. The secondary stream may be largerxe2x80x94that is, the pipe diameter for the secondary stream may be greater than that of the primary stream, and/or the flow of total fluid may be greater in the secondary stream, but we prefer that the secondary streamxe2x80x94the stream containing a higher ratio of liquid to gasxe2x80x94be of a smaller diameter than the primary stream. We use the terms xe2x80x9csecondary stream,xe2x80x9d xe2x80x9cbypass,xe2x80x9d and xe2x80x9cslip streamxe2x80x9d interchangeably. There may be more than one secondary or slip stream. Further, the primary stream may be directed to another splitter to be further split into additional streams, at least one having an enhanced ratio of liquid to gas compared to the fluid entering it, for additional injection into elevated regions of the reactor, preferably above the product withdrawal level. However, fluidization of the bed 2 (FIG. 1) must be maintained throughout; fluidization requires a sufficient quantity and velocity of fluid through line 3 to distribution plate 7.
We use the term xe2x80x9cthrough a direct passagexe2x80x9d to mean that the slip stream is passed directly from the elbow, bend, tee, or other conduit segment (splitter) to the reaction zone of the reactor, or to the upstream end of another splitter, without going through any mechanical equipment such as separators, hydrocyclones, demisters, scrubbers, entrainment collection devices, pumps, compressors or atomizers.
The ability to pass the slip stream through a direct passage into the reaction zone of the fluidized bed is enhanced by the usual slight reduction in pressure in the fluidized bed from its lower region to its upper region. Commonly, the pressure in the upper regions is from 0.04 to 0.15 psi per foot of height less than that in the lower regions of the bed. Thus, the higher the injection point in the bed, the greater will be the difference between the pressure in the slip stream and that in the reactor bed, which of course assists the flow of the secondary stream into the fluidized bed. Generally, we will inject the secondary stream at one or more points between six inches and 10 feet above the distributor plate of a commercial polyolefin reactor such as that shown in FIG. 1, but we prefer to inject the secondary stream at a height between eighteen (18) inches and ninety-six (96) inches above the distributor plate. Recycle injection is preferably above the point of product withdrawal.
Our invention includes a conduit segment which will provide a slip stream through a direct passage from a preferred elbow configuration defining a settling chamber and a discharge duct for the slip stream located at the bend of the elbow. More particularly, our invention includes a splitter for splitting a partially condensed recycle stream from a recycle stream in a fluidized bed polyolefin reactor, the splitter comprising an inlet portion, a primary outlet portion communicating with the curved portion, a secondary outlet portion, the secondary outlet portion preferably including a settling chamber located downstream from, adjacent to, and on the outside radius of said curved portion, and a slip stream conduit communicating with the settling chamber, the slip stream conduit preferably having a smaller effective diameter than that of the primary outlet portion and of the settling chamber.
This invention is an improvement in the xe2x80x9ccondensed modexe2x80x9d of operation. As disclosed by Jenkins, et al. in U.S. Pat. Nos. 4,543,399 and 4,588,790 and by DeChellis, et al. in U.S. Pat. No. 5,352,749, operation in xe2x80x9ccondensed modexe2x80x9d permits an increase in the space time yield of the reaction systemxe2x80x94that is, the amount of polymer produced per unit of time in a given fluidized bed reactor volume. Also disclosed by DeChellis in the aforementioned U.S. patent is that excessively high levels of liquid introduced to the fluidized bed may promote the formation of undesirable polymer agglomerates, the presence of which can lead to bed collapse and reactor shutdown. Excessive liquids can also influence local bed temperatures which yield undesirable inconsistencies in polymer product properties.
Our invention provides an increase in the space time yield (polymer production per unit of time) of a reaction system of a given volume, compared to other condensed mode techniques. In particular, the separation of the partially condensed recycle stream is accomplished without the use of costly mechanical separating devices such as separators, hydrocyclones, demisters, scrubbers, entrainment collection devices, pumps, compressors or atomizers.
An important aspect of our invention is that the injection of liquid above the point of product withdrawal reduces the quantity of liquid carried out of the fluidized bed with the solid and gas during the discharge process. This improves the product removal capacity and the raw material efficiency of the semi-batch discharge process. Regulation of the liquid split as a function of monitored or modeled liquid in the discharge tanks enhances the efficiency of the system.